The present invention generally relates to a collector for collecting liquid samples from a source of liquid. More particularly, the present invention relates to a collector for collecting samples of liquid from at least one semiconductor wafer cleaning bath so that automated analytical instrumentation can analyze the samples for contamination.
Semiconductor wafers suitable for the fabrication of integrated circuits are produced by slicing thin wafers from a single crystal silicon ingot. After slicing, the wafers undergo a lapping process to give them a substantially uniform thickness. The wafers are then etched to remove damage and produce a smooth surface. The next step in a conventional wafer shaping process is a polishing step to produce a highly reflective and damage-free surface on at least one face of the wafer. It is upon this polished face that electrical device fabrication takes place.
The presence of contaminants on the surface of a semiconductor wafer can greatly diminish the quality and performance of integrated circuits fabricated from the wafer. Thus, semiconductor wafers are typically cleaned after some or all of the above wafer preparation steps to help reduce the amount of contamination present on the final wafer product. For example, organic and other particulate contaminants can be removed from the surfaces of a single crystal silicon wafer by immersing the wafer in each of a series of cleaning bath solutions.
Monitoring the concentration of contaminants in a semiconductor cleaning bath solution serves several useful purposes. First, the useful life of the cleaning bath solution can be maximized. As wafers are continuously processed through a particular cleaning bath, the concentration of contamination in the cleaning bath solution will generally rise. Ultimately, the cleaning bath solution becomes saturated with contamination, and will no longer adequately clean the wafer. By monitoring the concentration of contaminants present in a particular cleaning bath solution, the point at which the solution is spent can be more accurately determined.
The second advantage realized by monitoring the concentration of contaminants in a cleaning bath solution is that sources of contamination can be more accurately identified. The various reagents used to make up the cleaning bath solution may contain impurities that actually contaminate the wafer, as opposed to cleaning it. The preparation of purer cleaning bath solutions requires the identification and elimination of these sources of contamination, which are more easily accomplished by monitoring the concentration of contaminants in a cleaning bath.
The accuracy and reliability of the analytical data obtained by the monitoring of cleaning baths greatly impact both the identification of the source of a contaminant and the determination of the useful life of a cleaning bath. In addition, because even low levels of some impurities can result in wafer surface contamination, the sensitivity of the analytical method is also critical.
Traditionally, the type and concentration of a contaminant in a cleaning bath solution have been monitored through the use of a manual sampling technique, commonly referred to as an "off-line" method of sampling, wherein a human operator collects a sample of the cleaning bath solution. The sample is then transported to a laboratory for analysis.
One disadvantage of this off-line sampling method is that it is prone to the introduction of additional contaminants from outside sources. For example, human contact with the sample can lead to the introduction of contaminants such as aluminum, iron, calcium, and sodium. In addition, the vial or container which holds the sample, as well as the pipette or other sampling device, typically cannot be sufficiently cleaned to avoid the introduction of outside contaminants into the sample. Thus, this off-line sampling method lacks the accuracy and sensitivity needed to provide representative results of the actual condition of the cleaning bath solution being tested.
Also, it can take up to several hours for a laboratory to complete the analysis of the sample and provide the results to the operators responsible for wafer cleaning. In the interim period, if wafer cleaning continues, a bath containing highly contaminated solution can produce hundreds of unacceptable wafers. This necessitates the recall and re-cleaning of these wafers. Alternatively, use of the bath could be halted while operators wait for the results. In either case, the net effect is an increase in production cost and a decrease in overall efficiency of the cleaning process.
To avoid the time consuming and potentially costly process of laboratory analysis, it has become common practice to simply discard the cleaning solution after a predetermined period of time, such as every 12 hours. However, the many variables which dictate the useful life of a cleaning bath solution, including the quality of the chemical reagents, the quality of the process water, the cleanliness of the wafers immersed in the solution, and the precautions taken to prevent contamination by human operators, are not taken into account under such a method. Therefore, without the benefit of an analysis for contaminants, a portion of the useful life of the bath may be wasted.
To overcome the above disadvantages, the concentration of impurities in a cleaning bath solution can be determined by an "on-line" process which allows for the sampling and analysis of the cleaning solution in a single, integrated process. That is, a sample is mechanically removed from a cleaning bath solution and automatically analyzed by analytical instrumentation without being directly exposed to an environment in which a human operator is present. An example of such a process is the co-assigned U.S. application, incorporated herein by reference, filed the same date as the present application, naming Larry W. Shive and Erik J. Mori as inventors, and entitled "Process for Monitoring the Concentration of Metallic Impurities in a Wafer Cleaning Solution."
A mechanical sampling and automated analysis system should preferably be constructed to ensure that the sample obtained from the cleaning bath is representative of the cleaning bath as a whole. If the sample obtained is not representative of the cleaning bath, accurate and reliable contamination measurements may not be obtained. To help ensure that the sample obtained is representative of the cleaning bath as a whole, the on-line system must not be a source of contamination itself. For instance, if valves were used in the on-line system to intermittently collect a sample, they may ultimately wear and introduce contamination into the sample that is not present in the cleaning bath as a whole, causing inaccurate contamination measurements.
Further, when multiple baths are to be sampled, it may be impractical to mechanically remove a sample from the cleaning baths themselves because the baths are typically positioned a significant distance away from each other. In this instance, a collection device may be needed to facilitate automated analysis. The collection device can maintain a portion of the cleaning bath solution from each bath in separate receptacles, all the receptacles being in close proximity to each other. If the collected portion of the cleaning solution is allowed to stagnate, however, the sample removed therefrom will not be representative of the cleaning bath as a whole and inaccurate contamination measurements will result.